Tube fitting and tube equipped with tube fitting

ABSTRACT

A tube fitting is provided with a resin coating layer over a coated region surfaces of a threaded portion and a contact portion. The resin coating layer includes a polyethylene based substance, a lubricant, and solid particles. When mass per unit area w (g/m2) is defined as a value obtained by dividing the mass difference between a state with the resin coating layer and a state without the resin coating layer by the surface area of the coated region, the mass per unit area w satisfies a relation 0.79&lt;w&lt;10.07.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Japanese Patent Application No.2019-45239, filed on Feb. 22, 2019 in the Japan Patent Office, the wholedisclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a tube fitting and to a tube equippedwith a tube fitting.

BACKGROUND ART

It is per se known to apply a lubricant and/or an adhesive to thesurface of a screw member such as a standardized bolt or nut, and toperform surface processing such as plating and so on thereupon, with theobject of preventing loosening of the screw member and enhancing theaxial force. Furthermore, a technique is per se known of applying aresin coating to a threaded tube fitting that is employed for anautomobile brake tube or the like. For example, refer to JapaneseLaid-Open Patent Publication 2015-230099, Japanese Laid-Open PatentPublication 2009-299895, and European Patent Application Publication2,706,277.

If the tightening torque for fastening a screw member to a mating memberis constant, then the axial force becomes greater, the lower is thefrictional force acting between the screw member and the mating member.Accordingly, surface treatment is performed upon the screw member, inorder to reduce the frictional force. This makes it possible to obtain ahigher axial force with the same tightening torque. This does not onlyhold for standardized screw members; the same is the case for threadedtube fittings. For example, tube fittings are used for coupling metallictubes.

SUMMARY OF INVENTION Technical Problem

When a tube fitting is used for coupling of a metallic tube, with thetube fitting installed on the external periphery of the tube, an annularportion termed an ISO flare or a double flare or the like is formed uponthe tube end of the tube. Since this annular portion projects radiallyoutward from the tube and is larger than the inner diameter of the tubefitting, accordingly the tube fitting is prevented from coming off thetube end of the tube by the annular portion. Moreover, since a tubeemployed for piping on an automobile is bent to follow the layout of thebottom portion of the vehicle, accordingly the tube fitting is alsoprevented from coming off in the direction away from the tube end by thebent portion of the tube. Due to this when, in order to performservicing or repair of some device to which the tube is coupled, thefastening state of the tube fitting is loosened and, after the tube hasbeen removed, it is returned back to the original state, it is necessaryto re-use the same tube fitting for each time of the serving or fitting,as long as the tube fitting is replaced together with the tube duringsuch servicing or repair.

When, in order to re-use the tube fitting, the tube fitting isrepeatedly fastened and released with the same tightening torque, therehas been a tendency for the axial force to decrease along with increaseof the number of repetitions. Since the axial force is less the greateris this axial force decrease ratio, accordingly, if the same tighteningtorque is employed for engagement when reusing a tube fitting, there isa possibility that it will not be possible to obtain the desiredcoupling force with the re-used tube fitting.

On the other hand, when the tube fitting is fastened, sometimes it mayhappen that co-rotation may take place, in which the tube rotatestogether with the tube fitting. The co-rotation occurs when thefrictional force generated between the annular portion provided to thetube and the tube fitting exceeds the frictional force generated betweenthe annular portion and the mating member. If the tube fitting isfastened while fixating the tube in order to prevent this co-rotation,then a co-rotation torque that twists the tube is generated as areaction force to prevent the co-rotation. If this co-rotation torque ishigh, then damage to the tube may take place. Furthermore, since thereaction force of the co-rotation torque acts in the direction to loosenthe tube fitting which has been fastened, accordingly, if theco-rotation torque remains in the state with the tube attached to thevehicle, then loosening of the tube fitting may be induced by vibrationof the vehicle. Accordingly, the upper limit value of the co-rotationtorque that is generated when fastening the tube fitting is determinedin consideration of the strength of the tube and in consideration ofvibration of the vehicle.

The co-rotation torque is at a maximum when the tube fitting is fastenedfor the first time, decreases the next time it is used, and tends not tochange very much with the number of times it is subsequently used. Dueto this, if the co-rotation torque is less than the upper limit value atthe first fastening, then the co-rotation torque will not exceed thisupper limit value when the tube fitting is re-used.

The present inventors have found that a factor affecting the axial forcedecrease ratio due to re-use of the tube fitting and the co-rotationtorque at the time of initial fastening is the thickness of the resincoating layer. Furthermore, the present inventors have found that thethickness of the resin coating layer is one of the factors that exertsinfluence upon the corrosion resistance of the tube fitting, and that,if the thickness of the resin coating layer is different, then therewill be a difference in the corrosion resistance of the tube fittingwhen it is re-used.

Accordingly, an aspect of the present invention is for solving a problemto provide a tube fitting and a tube equipped with a tube fitting thatare capable of providing an initial axial force that makes theco-rotation torque less than an upper limit value and of keeping theaxial force decrease ratio low when the tube fitting is repeatedlyfastened and released. Further, an aspect of the present invention isfor solving a problem to provide a tube fitting and a tube equipped witha tube fitting that are capable of ensuring corrosion resistance uponre-use that is equivalent to the corrosion resistance upon initialfastening. The above descriptions about the problems do not prevent fromthe presence of the other problems. It should be understood that it isnot necessary that each of the aspects of the present invention solvesall of the above problems.

SOLUTION TO TECHNICAL PROBLEM

A tube fitting according to an aspect of the present invention isprovided with a resin coating layer over a coated region includingsurfaces of a threaded portion and a contact portion. The resin coatinglayer includes a polyethylene based substance, a lubricant, and solidparticles. When mass per unit area w (g/m²) is defined as being a valueobtained by dividing the mass difference between a state with the resincoating layer and a state without the resin coating layer by the surfacearea of the coated region, the mass per unit area w satisfies a relation0.79<w<10.07. This relation is satisfied under conditions that anexternal thread of the threaded portion has an outer diameter of 9.53 to14.0 mm, and the contact portion has an inner diameter of 4.98 to 8.44mm, with respect to the tube fitting.

A tube equipped with a tube fitting according to an aspect of thepresent invention includes a tube made from metal and the tube fittinginstalled upon an external periphery of the tube. The tube is providedwith an annular portion and a bent portion to prevent the tube fittingfrom coming off. The tube fitting is provided with a resin coating layerover a coated region that includes surfaces of a threaded portion and acontact portion. The resin coating layer includes a polyethylene basedsubstance, a lubricant, and solid particles. When mass per unit area w(g/m²) is defined as being a value obtained by dividing a massdifference between a state with the resin coating layer and a statewithout the resin coating layer by a surface area of the coated region,the mass per unit area w satisfies a relation 0.79<w<10.07. Thisrelation is satisfied under conditions that an external thread of thethreaded portion has an outer diameter of 9.53 to 14.0 mm, and thecontact portion has an inner diameter of 4.98 to 8.44 mm.

A tube fitting according to an aspect of the present invention isprovided with a resin coating layer over a coated region includingsurfaces of a threaded portion and a contact portion. The resin coatinglayer includes a polyethylene based substance, a lubricant, and solidparticles. When a predetermined fastening test is repeated n times(where 1<n<6), if a maximum axial force generated in a first performanceof the fastening test is termed an initial axial force F₁ (kN) and amaximum axial force generated in the n-th performance of the fasteningtest is termed the n-th axial force F_(n) (kN), and if a value obtainedby −(F_(n)−F₁)/(n−1) is defined as being an axial force decrease ratio α(kN/turn), then a range of the mass per unit area w is set so as tosatisfy relations: F1<14.0 and 0<α<1.75. This relation is satisfiedunder conditions that an external thread of the threaded portion has anouter diameter of 9.53 to 14.0 mm, and the contact portion has an innerdiameter of 4.98 to 8.44 mm.

It should be understood that, in each aspect of the present invention,the surface of the threaded portion means the surface in the range wherethe screw thread is formed that is actually engaged with the internalthread of the mating member, or that is scheduled to engage with thatinternal thread. Moreover, the surface of the contact portion means thecontact surface that actually contacts the annular portion or thecontact surface that is scheduled to contact the annular portion. And“including the surfaces of the threaded portion and the contact portion”means including all or a part of the surface of the threaded portion,and including all or a portion of the surface of the contact portion. Inaddition, the resin coating layer may be made by adhering coatingmaterial including the polyethylene based substance, the lubricant, andthe solid particles to the coated region,

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a figure showing a state when a plurality of brake tubes thathave been processed by installation of tube fittings and by bending areassembled together;

FIG. 2 is a figure showing a flare nut, which is an example of a tubefitting;

FIG. 3 is a figure showing a brake tube, on whose tube end is formed anISO flare which is an example of an annular portion;

FIG. 4 is a figure showing a state in which a brake tube is coupled to amaster cylinder, which is an example of a mating member;

FIG. 5 is an enlarged sectional view of a portion V of FIG. 2 ;

FIG. 6 is a figure showing a summary of a method for formation of aresin coating layer;

FIG. 7 is a figure showing a flare nut, which is another example of atube fitting;

FIG. 8 is a figure showing a brake tube, on whose tube end is formed adouble flare which is another example of an annular portion;

FIG. 9 is a sectional view showing a portion of a master cylinder, whichis an example of a mating member to which the brake tube of FIG. 5 iscoupled;

FIG. 10 is an enlarged sectional view of a portion X of FIG. 7 ;

FIG. 11A is a figure showing a list table of test samples;

FIG. 11B is a figure showing a list table following on from FIG. 11A;

FIG. 12 is a figure showing the structure of an axial force measurementdevice;

FIG. 13A is a figure showing test results of a fastening test;

FIG. 13B is a figure continuing on from FIG. 13A;

FIG. 14A is a figure in which results of evaluation are summarized andare classified into passed samples and rejected samples;

FIG. 14B is a figure continuing on from FIG. 14A; and

FIG. 15 is a figure showing a relationship between mass per unit areaand corrosion resistance.

DESCRIPTION OF EMBODIMENTS

As one example, the brake tubes of an automobile are employed asconduits that transmit fluid pressure generated by a master cylinder tobrake units that are provided to each of the vehicle wheels. In manycases, an ABS unit and/or an ESC unit are provided between the mastercylinder and the brake units, and brake tubes are also employed toconnect between these units. A plurality of brake tubes having differentdiameters may be selected according to various conditions such aspressure resistance and so on required between these units.

As shown in FIG. 1 , a group of brake tubes BT are processed by beingbent according to the layout of the underside of an automobile, aregathered together and held by, for example, clamps C made from resin,and are supplied to an automobile assembly line as an integratedcomponent for assembly. In order to stand up to the operating pressureof the brakes, each of the brake tubes BT is made as a double-walledwrapped tube made from metal plate such as steel sheet or the like,which has excellent pressure capacity. In the case of a brake tube, asone example, a tube with outer diameter φ from 4.76 to 8.00 mm may beselected. A flare nut FN is installed on each of the brake tubes BT, theflare unit FN being suited for an outer diameter of the brake tube BT.On the automobile assembly line, the operator implements coupling of thebrake tubes BT collectively by fastening the flare nuts FN installedupon the brake tubes BT to each of the units mentioned above with acommon tightening torque that is predetermined.

Tube end processing for high pressure is performed upon the tube end ofeach of the brake tubes BT in its state with its corresponding flarenuts FN installed upon it. As tube end processing for high pressure,there is tube end processing in which an ISO flare as prescribed by theInternational Organization for Standardization (ISO) is formed, or anannular portion Rp such as a double flare or the like as prescribed bythe Japan Automobile Manufacturers Association (JASO) is formed. Withrespect to each of the brake tubes BT, a tube end processing and abending processing are implemented. In the tube end processing, theannular portions Rp are formed in the state in which the flare nuts FNare installed on the periphery of the brake tube BT, and in the bendingprocessing bent portions Bp are formed upon the brake tube BT. Due tothis, the flare nuts FN are prevented from coming off from the braketube BT by the annular portions Rp, and by the bent portions Bp that areprovided in positions remote from those annular portions Rp.

The First Embodiment

FIG. 2 shows a flare nut 1A that is suitable for an ISO flare. Thisflare nut 1A corresponds to an example of the “tube fitting” of thepresent invention. The flare nut 1A is a hollow tube fitting that isformed with a through hole 10 through which a tube can be inserted. Theflare nut 1A includes a threaded portion 12 upon which an externalthread 12 a is formed, a head portion 13 that is provided at one endside of the threaded portion 12, and a contact portion 14 that isprovided at the other end side of the threaded portion 12. The headportion 13, the threaded portion 12, and the contact portion 14 are allpenetrated by the through hole 10 that extends in the direction of thecenter line CL1. The through hole 10 of the shown flare nut 1A is formedto have an inner diameter that is constant along its axial direction,but this could be varied; for example, instead of this through hole 10,there could be provided a through hole having the shape of a steppedhole whose inner diameter changes at a predetermined location in itsaxial direction.

As one example, the external thread 12 a formed upon the threadedportion 12 may be a standard ISO metric coarse thread, and is engagedwith a corresponding internal thread 12 b formed in the mating member(refer to FIG. 4 ). However, as another example, this external thread 12a could be changed to a metric fine pitch thread according to the samestandard. Since the lead angle of a fine pitch thread is small comparedto that of a coarse thread, accordingly, by changing to a fine pitchthread, it is possible to provide a flare nut that is less likely toloosen under the same axial force. Moreover, with regard to the size ofthe threaded portion 12 of the flare nut 1A to be applied to the braketube BT described above, there is a tendency for the size of the screwemployed to be larger, the larger is the outer diameter of the tube tobe installed. And, with regard to the size of the threaded portion 12,except in special circumstances, its nominal diameter is generally fromM10 to M14, in other words its outer diameter is generally in the rangeof 10.0 to 14.0 mm. However, if an inch thread is provided upon theflare nut 1A, then a thread with nominal diameter in the range of ⅜″ to½″ (around 9.53 to 12.7 mm) is employed. Accordingly, the outer diameterof the external thread 12 a that can be employed for the flare nut 1A isin the range of 9.53 to 14.0 mm.

The head portion 13 is a location where a tightening torque is inputtedduring fastening, and has the shape of a standardized hexagon so that itcan be fastened with a conventional tool such as a flare nut wrench orthe like. The size of the head portion 13 is selected to match the sizeof the threaded portion 12, but, unlike the case with the head portionof a standardized bolt, it may be communalized to some extent in orderto reduce the number of tool changes.

The contact portion 14 is provided at the end portion of the flare nut1A along the center line CL1 at the right side of FIG. 2 , or, to put itin another manner, is provided at the end portion of the flare nut 1A inthe direction of progression of the external thread 12 a when the flarenut 1A is being fastened. When the flare nut 1A is being fastenedagainst the mating member, the contact portion 14 has the function ofwhile contacting against an annular portion 16 that is formed as an ISOflare on the brake tube BT (refer to FIG. 3 ), pressing that annularportion 16 against the mating member. In the case of the flare nut 1Ashown in FIG. 2 , the contact portion 14 includes a cylindrical portionthat extends from the threaded portion 12 to the end of the flare nut1A. And a chamfered portion 10 a is provided at the boundary portionbetween the contact portion 14 and the through hole 10, and has aninclination of about 45° with respect to the direction of the centerline CL1. Due to this chamfered portion 10 a, interference between thetube exterior and the flare nut 1A at the time of fastening and stressconcentration at the boundary portion between the through hole 10 andthe contact portion 14 are mitigated. It should be noted that thechamfered portion 10 a has a conical surface in which the ridge linesappearing in a cross section containing the center line CL1 are straightlines. However, instead of this chamfered portion 10 a, it would also beacceptable to change this region to a processed portion having a curvedsurface whose ridge lines consist of curves described by one or aplurality of circular arcs convex toward the center.

The inner diameter of the contact portion 14 is determined by the innerdiameter of the through hole 10. If, for example, the outer diameter φof the brake tube BT is 4.76 mm, then the inner diameter d of thecontact portion 14 may be set to 4.98 mm; if the outer diameter φ is 6.0mm, then the inner diameter d may be set to 6.24 mm; if the outerdiameter φ is 6.35 mm, then the inner diameter d may be set to 6.59 mm;and, if the outer diameter φ is 8.0 mm, then the inner diameter d may beset to 8.29 mm. For example, an error of +0.15 mm in the inner diameterd of the contact portion 14 may be permitted. Accordingly, the contactportion 14 employed for the flare nut 1A can have a range of innerdiameter d of 4.98 to 8.44 mm.

As shown in FIG. 3 , the annular portion 16 is formed at the tube end ofthe brake tube BT. As one example of a method for forming this annularportion 16, first, a resin coating layer BTa on the brake tube BT isdetached from the tube end of the brake tube BT over a predeterminedrange in the longitudinal direction of the tube axis Tx around itscircumferential direction, and then the annular portion 16 made in theshape of an ISO flare is formed upon the end portion of a portion BTbfrom which the resin coating layer BTa has thus been detached, so as toproject in the radially outward direction orthogonally to the tube axisTx. In some cases, depending upon the nature of the resin material fromwhich the resin coating layer BTa is made, the annular portion 16 maysimply be formed upon the end portion of the brake tube BT withoutparticularly detaching the resin coating layer BTa.

As an example of use of this flare nut 1A, a case in which the braketube BT is coupled to a master cylinder MC1 will now be explained withreference to FIG. 4 . The master cylinder MC1, which is an example of amating member, has a housing 40. An insertion hole 41 is formed in thehousing 40, and the brake tube BT is inserted thereinto. The insertionhole 41 opens to the exterior of the housing 40, and its end opposite toits opening portion communicates with a fluid passage 42 formed in thehousing 40. The fluid passage 42 opens to the bottom portion 43 of theinsertion hole 41. This bottom portion 43 is formed in a recessed shapeinto the interior side of the housing 40 to match the shape of theannular portion 16 on the brake tube BT. An internal thread 12 b thatengages with the external thread 12 a of the flare nut 1A is formed onthe inner peripheral surface of the insertion hole 41 formed in thehousing 40.

First, in the state with the flare nut 1A shifted back from the tube endof the brake tube BT, the brake tube BT is inserted so that the annularportion 16 of the brake tube BT abuts the bottom portion 43 of insertionhole 41. In this state, the flare nut 1A is approached close to theinsertion hole 41, so that the external thread 12 a of the threadedportion 21 and the internal thread 12 b of the housing 40 engagetogether. When the flare nut 1A is rotated in the fastening direction toa sufficient extent, its contact portion 14 contacts against the annularportion 16. And, when the flare nut 1A is further tightened up while itscontact portion 14 is in contact with the annular portion 16, theannular portion 16 is pressed against the bottom portion 43 by thecontact portion 14. While the flare nut 1A is thus being tightened, theannular portion 16 is sandwiched between the contact portion 14 and thebottom portion 43, and gradually deforms while transitioning fromelastic deformation to plastic deformation. Due to this, the brake tubeBT is coupled to the master cylinder MC1 in a liquid-tight manner. Thecoupling force for the brake tube BT is determined by the maximum axialforce that acts during this type of fastening operation.

In order to couple the brake tube BT firmly, as shown in FIG. 5 , aresin coating layer 18 is provided upon the flare nut 1A so as toincrease or stabilize the axial force when it is fastened. The flare nut1A has a surface 17 where a zinc based plated layer P1 is formed on ametallic base M1, and the resin coating layer 18 is provided upon thissurface 17. This zinc based plated layer P1 is principally provided inorder to enhance the corrosion resistance. One of zinc plating,zinc-iron alloy plating, or zinc-nickel alloy plating may be performedin order to form the zinc based plated layer P1. In this embodiment, azinc-nickel alloy plated layer is provided as the zinc based platedlayer P1.

The resin coating layer 18 is formed on, at least, a coated region Rthat includes the surfaces of the threaded portion 12 and the contactportion 14 (refer to FIG. 2 ). In this embodiment, the coated region Ris set over the entire surface of the flare nut 1A. In other words, thecoated region R is set over the surfaces of the threaded portion 12, thehead portion 13, and the contact portion 14 of the flare nut 1A, andover the inner circumferential surface of the flare nut 1A which thethrough hole 10 is pierced through. The resin coating layer 18 is formedby adhering, to the coated region R, a coating material C that includesas components a polyethylene based substance, a lubricant, and solidparticles, and whose viscosity is adjusted to a predetermined level. Theresin coating layer 18 includes the polyethylene based substance, thelubricant, and the solid particles. For example, polyethylene or apolyethylene copolymer may be selected for the polyethylene basedsubstance. And, for example, one or any combination of polyethylene wax,molybdenum disulfide, graphite, or boron nitride may be selected for thelubricant. The lubricant may be solid, or may be liquid. Moreover, forexample, one or any combination of silicon dioxide, silicon nitride, ortitanium nitride may be selected for the solid particles.

It would also be acceptable for the resin coating layer 18 to beprovided upon the surface 17 where the zinc based plated layer P1 hasbeen subjected to chemical conversion treatment. To put it in anothermanner, it would be acceptable to arrange for a chemical conversiontreatment layer to be present between the zinc based plated layer P1 andthe resin coating layer 18. Due to this, the adherence between thesurface 17 and the resin coating layer 18 is enhanced. Metallic atomsselected from titanium, zirconium, molybdenum, tungsten, vanadium,manganese, nickel, cobalt, chromium, and lead may be included in thischemical conversion treatment layer. Moreover, some of these metallicatoms may be included in the chemical conversion treatment layer ascompounds such as oxides or the like. The chemical conversion treatmentlayer may be a chromium-free chemical conversion treatment layer. Thechemical conversion treatment process for forming the chemicalconversion treatment layer may be a reaction type process or anapplication type process, and may be a trivalent chromium chemicalconversion treatment or a chromium-free chemical conversion treatment.It should be understood that the frictional coefficient of the resincoating layer 18 is smaller than the frictional coefficient of thesurface 17 where the zinc based plated layer P1 or the chemicalconversion treatment layer is formed.

One example of a method for forming the resin coating layer 18 will nowbe explained with reference to FIG. 6 . First, in a preparation step A,an untreated nut 1′ upon which the plating processing described abovehas been performed but upon which no resin coating layer 18 is formed isprepared. A plural number of these untreated nuts 1′ may be prepared.These prepared untreated nuts 1′ are put into a processing basket 30that has a mesh of a predetermined size. The next coating step B isperformed with the untreated nuts 1′ held in this processing basket 30.

As one example of the coating step B, a dip coating method may beimplemented. The coating step B includes a dipping step b1 of dippingthe untreated nuts 1′ into a coating material C, and a drying step b2 ofdrying the coating material C that is adhered to the untreated nuts 1′.

In the dipping step b1, the processing basket 30 containing theuntreated nuts 1′ is submerged from above into a dipping bath 31 inwhich coating material C is held to a predetermined level. The dippingbath 31 has the function of regulating the temperature of the liquidheld in the dipping bath 31. As one example, the temperature of thecoating material C may be regulated to be within the range of 30° C. to40° C. A sufficient amount of the coating material C is contained in thedipping bath 31 so that it is possible to ignore change of thetemperature of the coating material C due to the untreated nuts 1′,which are the objects to be processed, being dipped thereinto.

Subsequently, the processing basket 30 that has been submerged into thedipping bath 31 is pulled up out from the dipping bath 31, and then thedrying step b2 is implemented. In this drying step b2, the processingbasket 30 with the untreated nuts 1′ held therein is put into a drier32. The drier 32 has a processing space 33 whose temperature can beadjusted to be within a predetermined temperature range, and a rotationmechanism 34 that rotates around its axial line Ax while holding theprocessing basket 30. In this drying step b2, the processing basket 30held by the rotation mechanism 34 is rotated at a predetermined speedwhile the temperature within the processing space 33 is held at thepredetermined temperature. The rotation mechanism 34 is capable ofchanging the rotational speed. For example, the rotational speed may becontrolled within the range of 100 rpm to 900 rpm. The time period forprocessing by the drier 32 is set as appropriate. As a result, excesscoating material C is dragged off by the centrifugal force of therotation mechanism 34, and the coating material C that adheres to thecoated regions R is dried and fixated. The flare nut 1A provided withthe resin coating layer 18 is manufactured by performing this dryingstep b2.

The thickness of the resin coating layer 18 provided upon the flare nut1A is controlled. As will be described hereinafter, the thickness of theresin coating layer 18 is understood to be a factor that exerts aninfluence upon the mechanical characteristics of the flare nut 1A, suchas its axial force and so on. Moreover, it is also understood that thethickness of the resin coating layer 18 also exerts an influence uponthe corrosion resistance of the flare nut 1A. For example, the thicknessof the resin coating layer 18 can be controlled by preparing a coatingmaterial C whose viscosity is regulated, and moreover by regulating thetemperature of the coating material C when the workpieces (i.e. theuntreated nuts) are immersed therein. Furthermore, the thickness of theresin coating layer 18 can also be controlled by, during the drying stepb2 described above, regulating the temperature within the processingspace 33 and the rotational speed of the rotation mechanism 34. In otherwords, the parameters that control the thickness of the resin coatinglayer 18 are the viscosity of the coating material C, the temperatureduring dipping, the temperature during drying, and the rotational speedof the rotation mechanism 34. It should be understood that, since thethickness of the resin coating layer 18 does not become uniform withoutany dependence upon the location upon the flare nut 1A, accordingly thethickness of the resin coating layer 18 is quantitatively controlled andmanaged by employing a mass per unit area that will be describedhereinafter as a physical quantity that is correlated with thisthickness.

The Second Embodiment

FIG. 7 shows a flare nut 1B that is suitable for a double flare (JASOflare). This flare nut 1B corresponds to an example of the “tubefitting” of the present invention. The flare nut 1B is a hollow tubefitting in which is formed a through hole 20 into which a tube can beinserted. The flare nut 1B includes a threaded portion 22 upon which anexternal thread 22 a is formed, a head portion 23 that is provided atone end side of the threaded portion 22, and a contact portion 24 thatis provided at the other end side of the threaded portion 22. The headportion 23, the threaded portion 22, and the contact portion 24 arepenetrated by the through hole 20 that extends in the direction of thecenter line CL2. In the case of this flare nut 1B, the through hole 20is formed to have an inner diameter that is constant along its axialdirection, but this could be varied; for example, instead of thisthrough hole 20, there could be provided a through hole having the shapeof a stepped hole whose inner diameter changes at a predeterminedlocation in its axial direction.

The external thread 22 a formed upon the threaded portion 22 has thesame specifications as the external thread 12 a provided upon thethreaded portion 12 of the flare nut 1A of the first embodiment: theouter diameter of the external thread 22 a that can be employed for thisflare nut 1B is in the range of 9.53 to 14.0 mm. Moreover, thespecification of the head portion 23 is also the same as thespecification of the head portion 13 of the flare nut 1A. And thespecification of the contact portion 24 is also the same as thespecification of the contact portion 14 of the flare nut 1A: the innerdiameter d of the contact portion 24 that can be employed for this flarenut 1B is in the range of 4.98 to 8.44 mm.

The contact portion 24 is provided at the end portion of the flare nut1B along the center line CL2 at the right side of FIG. 7 , or, to put itin another manner, is provided at the end portion of the flare nut 1B inthe direction of progression of the external thread 22 a when the flarenut 1B is being fastened. When the flare nut 1B is being fastening tothe mating member, the contact portion 24 has the function of whilecontacting against an annular portion 26 that is formed as a doubleflare on the brake tube BT (refer to FIG. 8 ), pressing that annularportion 26 against the mating member. In the case of the flare nut 1Bshown in FIG. 7 , the contact portion 24 is provided in the vicinity ofthe very end of the threaded portion 22, and no clearly definedcylindrical portion like the contact portion 14 of the flare nut 1A ispresent. However, there also exist some flare nuts suitable for doubleflare installation with a clearly defined cylindrical portion like thecontact portion 14 of the flare nut 1A. The contact portion 24 isprovided with a contact surface 24 a that is formed as a conical surfacehaving an inclination of about 42° with respect to the axial direction.

As shown in FIG. 8 , the annular portion 26 is formed at the tube end ofthe brake tube BT. As one example of a method for forming this annularportion 26, first, a resin coating layer BTa on the brake tube BT isdetached from the tube end of the brake tube BT over a predeterminedrange in the longitudinal direction of the tube axis Tx around itscircumferential direction, and then the annular portion 26 made in adouble flare shape is formed upon the end portion of a portion BTb fromwhich the resin coating layer Bta has thus been detached, so as toproject in the radially outward direction orthogonally to the tube axisTx. In some cases, depending upon the nature of the resin material fromwhich the resin coating layer BTa is made, the annular portion 26 maysimply be formed upon the end portion of the brake tube BT withoutdetaching the resin coating layer BTa.

An insertion hole 51 having the structure shown in FIG. 9 is formed in amating member to which the brake tube BT formed with the annular portion26 is to be coupled. For example, the insertion hole 51 may be formed ina master cylinder MC2, which is an example of such a mating member. Theinsertion hole 51 opens to the exterior of a housing 50, and its endopposite to its opening portion communicates with a fluid passage 52formed in the housing 50. The fluid passage 52 opens to the bottomportion 53 of the insertion hole 51. This bottom portion 53 is formed ina shape that projects toward the exterior, so as to match the shape ofthe annular portion 26 on the brake tube BT. An internal thread 22 bthat engages with the external thread 22 a of the flare nut 1B is formedon the inner peripheral surface of the insertion hole 51 formed in thehousing 50. The way in which the brake tube BT is coupled by using theflare nut 1B is the same as in the case described above of the flare nut1A, and accordingly explanation thereof will be omitted.

As shown in FIG. 10 , a resin coating layer 28 is provided upon theflare nut 1B. The flare nut 1B has a surface 27 where a zinc basedplated layer P2 is formed on a metallic base M2, and the resin coatinglayer 28 is provided upon this surface 27. This zinc based plated layerP2 is principally provided in order to enhance the corrosion resistance.One of zinc plating, zinc-iron alloy plating, or zinc-nickel alloyplating may be performed in order to form the zinc based plated layerP2. In this embodiment, a zinc-nickel alloy plated layer is provided asthe zinc based plated layer P2. And, in a similar manner to the casewith the first embodiment, it would also be acceptable to arrange for achemical conversion treatment layer to be present between the zinc basedplated layer P2 and the resin coating layer 28. Moreover, in a similarmanner to the case with the first embodiment, the frictional coefficientof the resin coating layer 28 is smaller than the frictional coefficientof the surface 27 where the zinc based plated layer P2 or the chemicalconversion treatment layer is formed. The way in which the resin coatinglayer 28 is formed is the same as in the case of the formation methodshown in FIG. 6 .

EXAMPLES

A: Fastening Tests

By repeatedly fastening the flare nuts 1A and 1B with the sametightening torque and then releasing them, there is a tendency for theaxial force at the moment of the tightening-up to decrease along withincrease of the number of repetitions. And, from the fastening testsdescribed below, it was found that the thickness of the resin coatinglayer is a factor affecting the axial force decrease ratio. It should beunderstood that only the fastening test results obtained for the flarenuts 1A are described below, since no significant difference was foundin the test results between the flare nuts 1A and the flare nuts 1B.

1. Test Samples

(1) Preparation of the Test Samples

As shown in FIGS. 11A and 11B, coating materials C1 through C4 of fourdifferent types were employed for forming resin coating layers uponflare nuts of three different types having different shapes and sizes,and these were classified into groups G1 through G12, so as to result ina total of twelve types. The dip coating method described above wasemployed as the method for forming the resin coating layers. Each of thegroups G1 through G12 includes a plurality of samples having differentresin coating layer thicknesses. As described above, the parameters thatcontrol the thickness of the resin coating layer are the viscosity ofthe coating material, the temperature during dipping, the temperatureduring drying, and the rotational speed of the rotation mechanismprovided in the drier. By varying these conditions, a plurality ofsamples were prepared for each of the groups G1 through G12, theplurality of samples having different resin coating layer thicknesses.The thickness of the resin coating layer was quantified using aparameter mass per unit area w (g/m²) described hereinafter, whichcorrelated with the thickness of the resin coating layer.

(2) Measurement of the Viscosities of the Coating Materials

In preparation of the coating materials, the viscosity of each of thecoating materials was measured by the following method.

Equipment used: A rotational viscometer compliant with the ISO 2555:1990standard.

(Device name: TVB-10M, made by Toki Sangyo Co., Ltd.)

Spindle rotational speed: 60 rpm

The viscosity at 25° C. was measured under the conditions describedabove.

(3) Viscosities of the Coating Materials

The viscosities of the coating materials C1 through C4 at 25° C. were asfollows:

Coating material C1: 4.51 mPa·s.

Coating material C2: 5.27 mPa·s.

Coating material C3: 4.25 mPa·s.

Coating material C4: 4.24 mPa·s.

(4) Components of the Coating Materials

Each of the coating materials C1 through C4 contained the polyethylenebased substance, the lubricant, and the solid particles described aboveas common components.

(5) Calculation of the Mass Per Unit Area

The thickness of the resin coating layer correlates with the mass ofsubstance adhering to the coated region. Accordingly, a value mass perunit area w (g/m²) obtained by dividing the difference in mass between astate with the resin coating layer and a state without the resin coatinglayer by the surface area of the coated region was defined as a physicalquantity correlated with the thickness of the resin coating layer. Thismass per unit area w was employed for quantifying the thickness of theresin coating layer.

The mass per unit area w was calculated by dividing the mass differencebetween the mass of the flare nut before the resin coating treatment andthe mass of the flare nut after formation of the resin coating layer, bythe total surface area of the flare nut. It should be understood that,conversely to the above method, it would also be possible to calculatethe mass per unit area w by dividing the mass difference between themass of the flare nut with the resin coating layer formed thereupon andthe mass of the flare nut after removal of the resin coating layer, bythe total surface area of the flare nut. As a method for removing theresin coating layer, for example, the method may be employed of dippingthe flare nut with the resin coating layer formed thereupon into anorganic solvent at high temperature, and then, after it has been dipped,washing the flare nut using an organic solvent separately prepared forwashing, and drying it. As the organic solvent into which the flare nutis dipped, for example, an organic solvent that is capable of dissolvingpolyethylene, such as benzene or decalin or the like, may be employed.The time intervals for this dipping and this drying may be set to such alevel that the flare nut from which the resin coating layer has beenremoved may be identified with the flare nut before the resin coatingtreatment. For example, the time period for immersion in the organicsolvent may be five hours, and, after washing with the organic solvent,the drying time period may be one hour. The flare nut from which theresin coating layer has been removed by the above treatment can beidentified with the flare nut before the resin coating treatment.

The total surface area of the flare nut was calculated on the basis ofthe design drawings for the flare nut, by employing a surface areacalculation function incorporated in CAD software. By employing thisfunction, the surface area can be calculated for flare nuts of anydesired range.

It should be understood that, although slight discrepancy occurs in thecalculated value of the surface area between CAD software, thisdiscrepancy is negligible in the calculation of the mass per unit areaw, which is calculated to two places of decimals. Moreover, anydiscrepancy between the value calculated by CAD software, and a valuecalculated on the basis of measurement data which is obtained bythree-dimensional measurement of the external dimensions of an actualtest piece, is at a level that may be ignored, in a similar manner.

(6) Numbering of the Samples

As shown in FIGS. 11A and 11B, sample numbers #101 to #421 were assignedto the samples, in order to distinguish them from one another. It shouldbe understood that the first digit of the sample number was assigned soas to correspond to the type of its coating material C1 through C4.

2. The Fastening Test Method

(1) An Axial Force Measurement Device

The axial force measurement device shown in FIG. 12 was employed formeasuring the axial force of each of the samples. FIG. 12 schematicallyshows the structure of this axial force measurement device 100. Atesting tube T, which corresponds to the brake tube BT, is set into theaxial force measurement device 100. The testing tube T is mounted in theaxial force measurement device 100 so that the axis Tx of this tube anda reference axis SAx coincide. A sample S of a flare nut is installedupon the testing tube T, and a testing annular portion TR is formed atthe end of the testing tube T. The axial force measurement device 100then performs tightening operation upon the sample S until apredetermined tightening torque is reached, and thereby couples thetesting tube T to a testing member TM, which corresponds to the matingmember. And the axial force measurement device 100 measures the axialforce and other physical quantities with respect to the sample S duringthe tightening operation.

The axial force measurement device 100 includes a frame 101, and, as oneexample, this frame 101 is installed on the floor portion or the like ofa test room. Each of a tightening actuation unit 102 that performs atightening operation upon the sample S, a mating member holding unit 103that holds the testing member TM, and a tube holding unit 103 that holdsthe testing tube T is provided to the frame 101 of the axial forcemeasurement device 100. The tightening actuation unit 102, the matingmember holding unit 104, and the tube holding unit 104 are provided tothe frame 101 so as to be aligned along the direction of the referenceaxis SAx.

The tightening actuation unit 102 includes a tool 110 that is fitted tothe head portion of the sample S, a motor 111 that rotationally drivesthe tool 110 around the reference axis SAx, and a tightening torquesensor 112 that outputs a signal corresponding to the rotational driveresistance of the tool 110.

The mating member holding unit 103 holds the testing member TM in afirst jig 103 a and a second jig 103 b that are separated in thedirection of the reference axis SAx. The testing member TM is separatedin the direction of the reference axis SAx, with one first part TMathereof being held in the first jig 103 a and another second part TMbthereof being held in the second jig 103 b. The first part TMa has ascrew hole 115 in which is formed an internal thread 115 b that engageswith the external thread on the sample S. And the second part TMb has abottom portion 116 against which the testing annular portion TR of thetesting tube T is pressed. When the screw hole 115 and the bottomportion 116 are abutted against one another concentrically, a hole shapeequivalent to the insertion holes 41, 51 described above is defined. Thefirst jig 103 a and the second jig 103 b can hold the screw hole 115 ofthe first part TMa and the bottom portion 116 of the second part TMb sothat they abut one another concentrically. The first jig 103 a is fixedto the frame 101. On the other hand, movement of the second jig 103 b inthe direction of the reference axis SAx is restricted in a state thatthe first part TMa and the second part TMb abut one another, and a loadcell 117 is interposed between the second jig 103 b and the frame 101 ofthe axial force measurement device 100.

The tube holding unit 104 includes a fixing mechanism 118 that clamps toa fixed position set at a predetermined distance (for example 0.3 m)from the end of the testing tube T, and a co-rotation torque sensor 119that outputs a signal corresponding to the torque around the referenceaxis SAx generated upon the fixing mechanism 118.

When the tightening operation is performed upon the sample S by thetightening actuation unit 101, the sample S engages with and screws intothe internal thread 115 b formed upon the first part TMa of the testingmember TM, and the testing annular portion TR is pushed against thebottom portion 116 formed on the second part TMb. Due to this, a forceis applied between the first part TMa and the second part TMb to pullthem apart from one another in the direction of the reference axis SAx.Due to the fact that the first part TMa is held in the first jig 103 awhich is fixed to the frame 101, it cannot shift in the direction of thereference axis SAx, while, due to the fact that the second part TMb isheld in the second jig 103 b, it is restricted from movement in thedirection of the reference axis SAx by the load cell 117 which isinterposed between the second jig 103 b and the frame 101. Thus, sincethe load imposed upon the second part TMb corresponds to the reactionforce of the axial force upon the sample S, accordingly the valuedetected by the load cell 117 can be treated as the measured value ofthe axial force. In other words, the axial force of the sample S can bemeasured directly on the basis of the output signal of the load cell117, without employing any calculation founded upon the tighteningtorque. The signals from the tightening torque sensor 112, the load cell117, and the co-rotation torque sensor 119 are inputted to a controldevice 120. As one example, the control device 120 may be a personalcomputer. The control device 120 performs predetermined processing uponthe input signals from the sensors, and stores as measurement results.data in which the axial force and the co-rotation torque are associatedwith the tightening torque inputted to the sample S, and moreover,according to requirements, may output those measurement results to anoutput device, for example to a display or the like.

(2) The Testing Method

Using the axial force measurement device 100 shown in FIG. 12 , anunused sample S was installed upon an unused testing tube T and thetightening operation above described above was performed, and thereafterrelease operation was performed by loosening the fastening of the samplein the tightening actuation unit 102 and by releasing the coupling ofthe testing tube T to the testing member TM. It was determined that thecoupling of the testing tube T was released when the value detected bythe load cell 117 returned to its initial value (for example 0.0 kN). Asone example, the tightening torque during fastening by the tighteningactuation unit 102 may be set to a value within the range of 12.0 Nm to22.0 Nm, for example to 17.0 Nm. The operation described above was thefirst fastening test, and similar fastening test (without exchanging thetesting tube T and the sample S) including the tightening operation, themeasurement of the axial force and so on, and the release operation wasrepeated a total of five times. In each fastening test, the axial forceand the co-rotation torque measured by the axial force measurementdevice 100 were acquired and recorded as measurement values. Theinterval between the fastening tests was, for example, 60 seconds.

It should be understood that the number of times the fastening test wasrepeated was determined on the basis of a limit for the number of timesthat the brake tube of an automobile is likely to be taken off andreplaced during the entire period of time from when the automobile isnew until it is finally scrapped. Although the chance for the brake tubeof an automobile to be taken off does not occur frequently, this limitwas estimated on the basis of a discrete probability distribution withthe number of times of removal being a random variable. Here, threeelements, that are the ABS unit, the master cylinder, and the brakeunit, were supposed to be the mating members to which the brake tube iscoupled, and, under the assumption that the brake tube and the flare nutmust necessarily be re-used in the event of a fault in any one of thesethree elements, the probability of the occurrence of a fault in any ofthese elements, the number of coupling points of the brake tube, theaverage value of the time period from when the vehicle is new to when itis finally scrapped, and other parameters were considered. Based uponthe above, it was considered that the probability of a brake tube beingtaken off six times or more than six times was so low as to benegligible. Accordingly, the number of times the fastening test wasrepeated was set to be less than six times, i.e. five times.

(3) The Axial Force Decrease Ratio

As a parameter for evaluation of the change of axial force due torepeated engagement testing performed upon the same flare nut, the axialforce decrease ratio α (kN/turn) is defined by the following Formula(1):α=−(F _(n) −F ₁)/(n−1)  (1)

Here F₁ (kN) is the initial axial force, which is the maximum axialforce that is generated during the first fastening test. And F_(n) (kN)is the n-th axial force, which is the maximum axial force that isgenerated during the n-th fastening test (where 1<n<6). However, Formula(1) is conditional upon the condition that the relation 0<F_(n)<F_(n-1)is satisfied, where the (n−1)-th axial force that is generated duringthe (n−1)-th fastening test is F_(n-1). Accordingly, α>0.

Since, as described above, in this testing the total number ofrepetitions was less than six, i.e. was five, accordingly the aboveFormula (1) that defines the axial force decrease ratio α may berewritten as the following Formula (1′), with the axial force during thefifth test being F₅:α=−(F ₅ −F ₁)/4  (1′)3. Test Results and Evaluation

The results of the fastening tests conducted as described above areshown in FIGS. 13A and 13B.

(1) The Evaluation Standard

Each of the samples was evaluated according to the following ReferenceStandards a and b, on the basis of the mechanical characteristics of theflare nuts. Reference Standard “a”: The initial axial force F₁ is lessthan 14.0 kN. Reference Standard “b”: The axial force decrease ratio αis less than 1.75 kN/turn.

Reference Standard “a” stipulates the upper limit value of the initialaxial force F₁. This upper limit value is determined on the basis of theupper limit value for the co-rotation torque. This upper limit value forthe co-rotation torque is determined in consideration of the strength ofthe tube and the vibration of the vehicle, and, for example, may be 1.0Nm. The co-rotation torque and the axial force are correlated, and theaxial force that corresponds to the upper limit value of the co-rotationtorque is uniquely determined. This axial force, for example, may be14.0 kN. The co-rotation torque is maximum at the time of firstfastening accompanying plastic deformation of the annular portion formedupon the tube and is reduced when the flare nut is re-used, but does nottend further to change much in dependence upon the number of times offurther re-use. Accordingly, due to the axial force matching ReferenceStandard “a”, it is guaranteed that the co-rotation torque will be lessthan the upper limit value even when the flare nut is re-used. It shouldbe understood that the lower limit value of the initial axial force F₁is set so that it is possible to ensure the required coupling force forthe tube, even if the axial force decreases when the flare nut isre-used. For example, it is preferable for the lower limit value for theinitial axial force F₁ to be 10.0 kN, more preferable for it to be 11.0kN, and yet more preferable for it to be 12.0 kN. In other words, it ispreferable for the initial axial force F₁ to satisfy 10.0<F₁<14.0, morepreferable for it to satisfy 11.0<F₁<14.0, and yet more preferable forit to satisfy 12.0<F₁<14.0.

Reference Standard “b” stipulates the upper limit value of the axialforce decrease ratio α. If, as one example, the axial force decreaseratio α is greater than or equal to 1.75 kN/turn, then, upon re-use, theaxial force will often drop below the lower limit value even if theflare nut is fastened with the same tightening torque as when it wasfastened for the first time. The lower limit value of this axial forceis set on the basis of the lower limit value for the coupling forcerequired for the brake tube. By conforming to Reference Standard “b”, itis possible to avoid the axial force dropping below the lower limitvalue even if, upon re-use of the flare nut, it is fastened with thesame tightening torque as when it was fastened for the first time. Thisensures the appropriate coupling force required for the brake tube evenwhen the flare nut is re-used. It should be understood that the smalleris the axial force decrease ratio α, the better, provided that α>0.

(2) The Results of Evaluation

The results of evaluation are shown in FIGS. 14A and 14B. In thesefigures, the samples are arranged in order from the smallest mass perunit area w to the largest, and passed samples that conform both toReference Standard “a” and Reference Standard “b” and rejected samplesfor which at least one of these Reference Standard “a” and ReferenceStandard “b” is not met are shown together. In FIGS. 14A and 14B, withrespect to respective Reference Standard “a” and Reference Standard “b”,cases for which the corresponding standard is met are shown by “y”,while cases for which the corresponding standard is not met are shown by“n”. Moreover, cases for which both Reference Standard “a” and ReferenceStandard “b” are met are shown by “YES”, while cases for which at leastone of Reference Standard “a” or Reference Standard “b” is not met areshown by “NO”.

(3) Considerations

As can be understood from FIGS. 14A and 14B, whether Reference Standard“a” or Reference Standard “b” is or is not met depends upon the mass perunit area w, irrespective of differences in the coating material. As themass per unit area w increases, generally there is a tendency for theinitial axial force F₁ to increase and for the axial force decreaseratio α to decrease. Conversely, as the mass per unit area w decreases,generally there is a tendency for the initial axial force F₁ to decreaseand for the axial force decrease ratio α to increase. However, it hasbeen found that, if the mass per unit area w is too large, thenReference Standard “a” is not met, while, if the mass per unit area w istoo small, then Reference Standard “b” is not met. When the passedsamples are reviewed, it is found that the mass per unit area w of thepassed samples is within the range 0.79<w<10.07. The lower is theinitial axial force F₁, the more it is possible to reduce damage to thetube while ensuring the required coupling force for the tube.Furthermore, when mass production of the flare nuts is considered, it isbeneficial in terms of production cost to keep the amount of coatingmaterial used as small as possible. Accordingly, when not only themechanical characteristics but also damage to the tube and theproduction costs of the flare nuts are considered, it is preferable forthe upper limit value of the mass per unit area w to be, for example,less than 9.00 g/m², more preferable for it to be less than 7.50 g/m²,and even more preferable for it to be less than 6.00 g/m². In otherwords, for the mass per unit area w, 0.79<w<9.00 is preferable,0.79<w<7.50 is more preferable, and 0.79<w<6.00 is even more preferable.

If the mass per unit area w is within any of the ranges described above,the mechanical characteristics required for the flare nut are satisfied.However, it has been found that, if the mass per unit area w is too low,although the mechanical characteristics for the flare nut are satisfied,there is a problem with the corrosion resistance of the flare nut whenit is re-used.

B: Corrosion Tests

Therefore, the relationship between the corrosion resistance of theflare nuts and the mass per unit area w was checked by corrosiontesting.

Preparation of the Samples

A predetermined number of flare nuts were prepared to perform thefastening test mentioned above upon the flare nuts and test samplegroups for which different numbers of trials were performed were createdin the following manner. The predetermined number of flare nuts havingthe same dimensions and shapes but with different values of mass perunit area w were prepared, and the fastening test described above wasperformed upon these flare nuts. By way of example, there were prepareda first-time sample group with which the fastening test described abovewas performed once and then the testing tube was taken off, a third-timesample group with which the fastening test described above was performedthree times and then the testing tube was taken off, and a fifth-timesample group with which the fastening test described above was performedfive times and then the testing tube was taken off. And, as acomparative example, an unused comparative sample group of flare nutswas prepared having the same dimensions and shapes as the above testsample groups, but different values of mass per unit area w.

Testing Method and Results

Corrosion testing was performed for each of the test sample groups andthe comparison sample group. This testing conformed to the SST (SaltSpray Test) stipulated by JASO 104-86. To summarize the test results,the relationship between corrosion resistance and mass per unit areashown in FIG. 15 was found. The time period (in hours) from the start ofthe test to the development of white rusting, which indicates thecorrosion resistance of the flare nut, is shown along the vertical axisin FIG. 15 . And the mass per unit area w, which is correlated with thethickness of the resin coating layer, is shown along the horizontal axisin FIG. 15 .

Considerations

As shown in FIG. 15 , it will be understood that the overall corrosionresistance tendency was for the corrosion resistance to be enhanced, thegreater was the mass per unit area w. And, when each test sample groupis compared with the comparison sample group, it is seen that, in theregions where the mass per unit area w was 1.20 or less, there was atendency for white rust to develop in shorter time periods for each ofthe test sample groups as compared with the comparison sample group,even though they had the same level of mass per unit area w. In otherwords, in the regions where the mass per unit area w was 1.20 or less,the corrosion resistance was reduced due to re-use. Moreover, when thetest sample groups in the same regions are compared with one another,there is a tendency for the corrosion resistance to be lower, thegreater was the number of repetitions. These tendencies were prominentwhen the mass per unit area w is in the range of 0.40 to 1.00.

On the other hand, it will be understood that, in the regions where themass per unit area w was greater than 1.20, there was no significantdifference in the corrosion resistance with respect to the mass per unitarea w for the test sample groups and the comparison sample group. Inother words, in the regions where the mass per unit area w was greaterthan 1.20, the corrosion resistance did not change according to thenumber of times of re-use. Accordingly, if the mass per unit area w isgreater than 1.20, then it is possible to ensure a corrosion resistancewhen the flare nut is re-used that is equivalent to the corrosionresistance when it is used for the first time. It should be understoodthat, while it is possible to ensure a corrosion resistance equivalentto when the flare nut is used for the first time if the mass per unitarea w is greater than 1.20, it is preferable for the mass per unit areaw to exceed 1.50 in order reliably to obtain corrosion resistance thatis not inferior to the corrosion resistance at the time of first use, itis more preferable for the mass per unit area w to exceed 1.80, and itis even more preferable for it to exceed 2.00.

C: Summary

By combining the results of the fastening tests and the results of thecorrosion resistance testing described above it is found that, in orderto ensure corrosion resistance of the flare nut upon re-use equivalentto the corrosion resistance at the first use while still satisfyingrequirements for the mechanical characteristics of the flare nut, it ispreferable that the following conditions are satisfied: the conditionobtained by the fastening tests is 0.79<w<10.07, and the conditionobtained by the corrosion resistance tests is 1.20<w. Accordingly, inrelation to the mass per unit area w, at least the following Formula (2)is satisfied:1.20<w<10.07  (2)

The present invention is not limited to the embodiments described above;it may be implemented in various forms. In the embodiments describedabove, the flare nuts were used upon brake tubes made from metal, butthe subject for use of the flare nuts is not limited to being braketube. For example, tubes of various types made from metal, such as vaportubes or the like, may also be employed as subjects. Each of the flarenuts 1A, 1B is only an example of a tube fitting that is used forcoupling to a metallic tube. The present invention can also be appliedto flare nuts having shapes different from those shown in the figures,provided that the external thread has an outer diameter of 9.53 through14.0 mm and the contact portion has an inner diameter of 4.98 through8.44 mm.

The coated region R according to the embodiments described above areprovided upon the entire surfaces of the flare nut, in other words uponthe entire surfaces of the threaded portion, of the head portion, and ofthe contact portion, and upon the entire inner circumferential surfaceof the flare nut which a through hole is pierced through. However, thefact that the coated region is provided over these entire surfaces isonly an example. For example, the coated region may be limited to beingprovided only upon the surface of the threaded portion and upon thesurface of the contact portion. In this case, the inner circumferentialsurface of the flare nut which a through hole is pierced through and thesurface of the head portion are excluded from the coated region.Furthermore, the coated region may not be always set upon the entiresurfaces of the threaded portion and the contact portion. For example,the coated region might be set only upon portions of the surfaces of thethreaded portion and the contact portion. In this case, for example, thecoated region could preferably be set upon an at least 40% portion ofthe surface of the threaded portion, more preferably could be set uponan at least 60% portion thereof, and even more preferably could be setupon an at least 80% portion thereof. Moreover, for example, the coatedregion could preferably be set upon an at least 40% portion of thesurface of the contact portion, more preferably could be set upon an atleast 60% portion thereof, and even more preferably could be set upon anat least 80% portion thereof. As described above, by the surface of thethreaded portion is meant the surface of the range over which the screwthread is formed that is actually engaged with the internal screw threadof the mating member, or that is scheduled to be engaged with thatinternal screw thread. Furthermore, by the surface of the contactportion is meant the contact surface that is actually in contact withthe annular portion, or that is scheduled to be in contact with theannular portion.

The embodiments described above are only examples in which the resincoating layer 18 is provided upon the surface 17 upon which the zincbased plated layer P1 is provided, and in which the resin coating layer28 is provided upon the surface 27 upon which the zinc based platedlayer P2 is provided. However, for example, it would also be possible toimplement the present invention by employing a metallic substrate uponwhich no chemical surface processing such as plating or the like hasbeen performed as the surface of a tube fitting, and by providing aresin coating layer upon that surface. Even with this arrangement, thefrictional coefficient of the resin coating layer will be smaller thanthe frictional coefficient of the surface of the metallic substrate.

Inventions that can be specified from the embodiments and variantembodiments explained above are described below.

A tube fitting according to an aspect of the invention disclosed is atube fitting that is capable of coupling a tube made from metal andprovided with an annular portion upon an end portion thereof, to amating member by being installed upon the external periphery of the tubeand by being fastened to the mating member in the state of contactingthe annular portion, the annular portion projecting in the radiallyoutward direction from the tube, the tube fitting including a threadedportion upon which is formed an external thread that engages with aninternal thread provided in the mating member; a contact portion that isprovided at the end portion of the tube fitting in the direction ofprogression of the external thread when the tube fitting is beingfastened, and that, when the tube fitting is fastened to the matingmember, contacts the annular portion to press the annular portionagainst the mating member; and a resin coating layer provided over acoated region that includes the surfaces of the threaded portion and thecontact portion; and wherein: a through hole extending in a directionparallel to the direction of progression is pierced through both thethreaded portion and the contact portion; the external thread has anouter diameter of 9.53 to 14.0 mm, and the contact portion has an innerdiameter of 4.98 to 8.44 mm; the resin coating layer includes apolyethylene based substance, a lubricant, and solid particles; and,when mass per unit area w (g/m²) is defined as being a value obtained bydividing the mass difference between a state with the resin coatinglayer and a state without the resin coating layer by the surface area ofthe coated region, the mass per unit area w satisfies a relation0.79<w<10.07.

According to the above aspect, the relation 0.79<w<10.07 is satisfiedwith respect to the mass per unit area w which correlates with thethickness of the resin coting layer provided to the tube fitting, andaccordingly, in a case that fastening and release are repeated, it ispossible to obtain an initial axial force which keeps a co-rotationtorque less than the upper limit value, and moreover it is possible tokeep the axial force decrease ratio low.

In the above aspect, a relation 1.20<w<10.07 may be satisfied withrespect to the mass per unit area w. According to this aspect, in a casethat fastening and release are repeated, it is possible to obtain aninitial axial force which keeps a co-rotation torque less than the upperlimit value, and moreover it is possible to keep the axial forcedecrease ratio low; and furthermore it is possible to ensure a corrosionresistance during re-use of the tube fitting that is equivalent to thecorrosion resistance upon first use.

In the above aspect, a relation 0.79<w<9.00, a relation 0.79<w<7.50, ora relation 0.79<w<6.00 may be satisfied with respect to the mass perunit area w. When mass production of flare nuts is considered, it isbeneficial in terms of production cost that the smaller the upper limitof the mass per unit area w is, the smaller the amount of coatingmaterial used becomes.

In the above aspect, when a testing member corresponding to the matingmember and a testing tube having the same outer diameter as the tube andhaving a testing annular portion corresponding to the annular portionare prepared, and when a fastening test is repeated n times (where1<n<6), the fastening test including: fastening operation to fasten thetube fitting to the testing member with a predetermined tighteningtorque in the state in which the contact portion is contacted againstthe testing annular portion; and release operation to release thecoupling of the testing tube from the testing member by loosening itsfastening state after the tightening operation, if the maximum axialforce generated in the first performance of the fastening test is termedthe initial axial force F₁ (kN) and the maximum axial force generated inthe n-th performance of the fastening test is termed the n-th axialforce F_(n) (kN), and if a value obtained by −(F_(n)−F_(i))/(n−1) isdefined as being the axial force decrease ratio α (kN/turn), then theaxial force decrease ratio α satisfies a relation 0<α<1.75. Since,according to this aspect, the relation 0<α<1.75 may be satisfied for theaxial force decrease ratio α. According to this aspect, it is possibleto suppress decrease of the axial force upon re-use, and it is possibleto obtain the desired coupling force during reuse even though the tubefitting is fastened with a tightening torque that is the same as thatemployed upon first use.

Further, the tube may be a brake tube that is employed as a brakeconduit for an automobile, and, when the predetermined tightening torqueis in the range of 12.0 to 22.0 Nm, the initial axial force F₁ maysatisfy a relation F₁<14.0. Since, in this case, it is guaranteed thatthe upper limit value for the co-rotation torque will not be exceeded,accordingly it is possible to prevent the induction of loosening of thetube fitting, while still avoiding causing damage to the brake tube.

In the above aspect, a zinc based plated layer may be formed upon thesurfaces of the threaded portion and the contact portion respectively,the resin coating layer may be provided over the zinc based platedlayer, and the frictional coefficient of the resin coating layer may besmaller than the frictional coefficient of the surfaces formed by thezinc based plated layer. According to this aspect, the corrosionresistance of the tube fitting is enhanced. In particular, if azinc-nickel alloy plated layer is provided as the zinc based platedlayer, then the enhancement of the corrosion resistance is particularlyprominent.

In the above aspect, the resin coating layer may be made by adheringcoating material including the polyethylene based substance, thelubricant, and the solid particles to the coated region, and theviscosity of the coating material may be in a range of 4.24 to 5.27mPa·s at a temperature of 25° C. measured using a rotationalviscosimeter conforming to the ISO 2555:1990 standard and the rotationalspeed of whose spindle may be set to 60 rpm. According to this aspect,by selecting a coating material whose viscosity comes within this sortof range, it is possible to control the mass per unit area w in anappropriate manner.

A tube equipped with a tube fitting according to an aspect of theinvention disclosed includes: a tube made from metal, the tube beingprovided with an annular portion upon an end portion thereof, theannular portion projecting in the radially outward direction from thetube, and provided with a bent portion at a position remote from theannular portion; and a tube fitting that is installed upon the externalperiphery of the tube so as to be prevented from coming off by theannular portion and the bent portion, and that can couple the tube to amating member by being fastened to the mating member in the state ofcontacting the annular portion; wherein the tube fitting includes: athreaded portion upon which is formed an external thread that engageswith an internal thread provided in the mating member; a contact portionthat is provided at the end portion of the tube fitting in the directionof progression of the external thread when the tube fitting beingfastened, and that, when the tube fitting is fastened to the matingmember, contacts the annular portion and presses the annular portionagainst the mating member; and a resin coating layer provided over acoated region that includes the surfaces of the threaded portion and thecontact portion; and wherein: a through hole extending in a directionparallel to the direction of progression is pierced through both thethreaded portion and the contact portion, the external thread has anouter diameter of 9.53 to 14.0 mm, and the contact portion has an innerdiameter of 4.98 to 8.44 mm; the resin coating layer is made by adheringa coating material including a polyethylene based substance, alubricant, and solid particles to the coated region; and, when mass perunit area w g/m²) is defined as being a value obtained by dividing themass difference between a state with the resin coating layer and a statewithout the resin coating layer by the surface area of the coatedregion, the mass per unit area w satisfies the relation 0.79<w<10.07.

According to the above aspect, the relation 0.79<w<10.07 is satisfiedwith respect to the mass per unit area w, and accordingly, in a casethat fastening and release are repeated, it is possible to obtain aninitial axial force which keeps a co-rotation torque less than the upperlimit value, and moreover it is possible to keep the axial forcedecrease ratio low.

In the above aspect, a relation 1.20<w<10.07 may be satisfied withrespect to the mass per unit area w. According to this aspect, in a casethat fastening and release are repeated, it is possible to obtain aninitial axial force which keeps a co-rotation torque less than the upperlimit value, and moreover it is possible to keep the axial forcedecrease ratio low; and furthermore it is possible to ensure a corrosionresistance during re-use of the tube fitting that is equivalent to thecorrosion resistance upon first use.

In the above aspect, the tube may be a brake tube that is employed as abrake conduit for an automobile, and, when a testing membercorresponding to the mating member and a testing tube having the sameouter diameter as the tube and having a testing annular portioncorresponding to the annular portion may be prepared, and when afastening test is repeated n times (where 1<n<6), the fastening testincluding: fastening operation to fasten the tube fitting to the testingmember with a tightening torque in the range of 12.0 to 22.0 Nm, in thestate in which the contact portion is contacted against the testingannular portion; and release operation to release the coupling of thetesting tube from the testing member by loosening its fastening stateafter the tightening operation: if the maximum axial force generated inthe first performance of the fastening test is termed the initial axialforce F₁ (kN) and the maximum axial force generated in the n-thperformance of the fastening test is termed the n-th axial force F_(n)(kN), and if a value obtained by −(F_(n)−F₁)/(n−1) is defined as beingthe axial force decrease ratio α (kN/turn), then the initial axial forceF₁ may satisfy a relation F₁<14.0 and the axial force decrease ratio αmay satisfy a relation 0<α<1.75.

Since, according to this aspect, the relation 0<α<1.75 may be satisfiedwith respect to the axial force decrease ratio α, accordingly it ispossible to prevent decrease of the axial force during re-use, and it ispossible to obtain the desired coupling force during re-use even whenfrightening is performed with the same tightening torque as thatemployed upon initial use. Moreover, since the relation F₁<14.0 issatisfied with respect to the initial axial force F₁, accordingly it ispossible to guarantee that the upper limit value for the co-rotationtorque is not exceeded, and it is possible to prevent induction ofloosening of the tube fitting while still avoiding damage to the tubethat is employed as a brake conduit.

In this aspect of the tube equipped with the tube fitting, a zinc basedplated layer may be formed upon the surfaces of the threaded portion andthe contact portion, the resin coating layer may be provided over thezinc based plated layer, a zinc-nickel alloy plated layer may beprovided as the zinc based plated layer, and the frictional coefficientof the resin coating layer may be smaller than the frictionalcoefficient of the surfaces formed by the zinc based plated layer.Since, according to this aspect, a zinc-nickel alloy plated layer isprovided as the zinc based layer, accordingly enhancement of thecorrosion resistance is prominent.

A tube fitting according to an aspect of the invention disclosed iscapable of coupling a tube made from metal and provided with an annularportion upon an end portion thereof, to a mating member by beinginstalled upon the external periphery of the tube and by being fastenedto the mating member in the state of contacting the annular portion, theannular portion projecting in the radially outward direction from thetube, the tube fitting comprising: a threaded portion upon which isformed an external thread that engages with an internal thread providedin the mating member; a contact portion that is provided at the endportion of the tube fitting in the direction of progression of theexternal thread when the tube fitting is being fastened, and that, whenthe tube fitting is fastened to the mating member, contacts the annularportion to press the annular portion against the mating member; and aresin coating layer provided over a coated region that includes thesurfaces of the threaded portion and the contact portion; and wherein:the tube is a brake tube that is employed as a brake conduit for anautomobile; a through hole extending in a direction parallel to thedirection of progression is pierced through both the threaded portionand the contact portion, the external thread has an outer diameter of9.53 to 14.0 mm, and the contact portion has an inner diameter of 4.98to 8.44 mm; the resin coating layer includes a polyethylene basedsubstance, a lubricant, and solid particles; and, when mass per unitarea w (g/m²) is defined as being a value obtained by dividing the massdifference between a state with the resin coating layer and a statewithout the resin coating layer by the surface area of the coatedregion, then, when a testing member corresponding to the mating memberand a testing tube having the same outer diameter as the tube and havinga testing annular portion corresponding to the annular portion areprepared, and a fastening test is repeated n times (where 1<n<6), thefastening test including: fastening operation to fasten the tube fittingto the testing member with a tightening torque in the range of 12.0 to22.0 Nm in the state in which the contact portion is contacted againstthe testing annular portion; and release operation to release thecoupling of the testing tube from the testing member by loosening itsfastening state after the fastening operation: if the maximum axialforce generated in the first performance of the fastening test is termedthe initial axial force F₁ (kN) and the maximum axial force generated inthe n-th performance of the fastening test is termed the n-th axialforce F_(n) (kN), and if a value obtained by −(F_(n)−F_(i))/(n−1) isdefined as being the axial force decrease ratio α (kN/turn), the rangeof the mass per unit area w is set so as to satisfy the relationsF₁<14.0 and 0<α<1.75.

Since, according to the above aspect, the relation 0<α<1.75 is satisfiedin relation to the axial force decrease ratio α, accordingly it ispossible to prevent decrease of the axial force during re-use, and it ispossible to obtain the desired coupling force during re-use even whenfastening is performed with the same tightening torque as that employedupon initial use. Moreover, since the relation F₁<14.0 is satisfied inrelation to the initial axial force F₁, accordingly it is possible toguarantee that the upper limit value for the co-rotation torque is notexceeded, and it is possible to prevent induction of loosening of thetube fitting while still avoiding damage to the tube that is employed asa brake conduit.

In the above aspect, the range for the mass per unit area w may be0.79<w<10.07. According to this tube fitting, when fastening and releaseare repeated, it is possible to obtain the initial axial force whilekeeping the co-rotation torque less than the upper limit value, andmoreover it is possible to keep the axial force decrease ratio low.

In the above aspect, a relation 1.20<w<10.07 may be satisfied withrespect to the mass per unit area w. According to this aspect, in a casethat fastening and release are repeated, it is possible to obtain aninitial axial force which keeps a co-rotation torque less than the upperlimit value, and moreover it is possible to keep the axial forcedecrease ratio low; and furthermore it is possible to ensure a corrosionresistance during re-use of the tube fitting that is equivalent to thecorrosion resistance upon first use.

In the above aspect, a relation 0.79<w<9.00, a relation 0.79<w<7.50, ora relation 0.79<w<6.00 may be satisfied with respect to the mass perunit area w. When mass production of flare nuts is considered, it isbeneficial in terms of production cost that the smaller the upper limitof the mass per unit area w is, the smaller the amount of coatingmaterial used becomes.

A method for manufacturing a tube fitting according to an aspect of theinvention disclosed is a method for manufacturing a tube fitting that iscapable of coupling a tube made from metal and provided with an annularportion upon an end portion thereof, to a mating member by beinginstalled upon the external periphery of the tube and by being fastenedto the mating member in the state of contacting the annular portion, theannular portion projecting in the radially outward direction from thetube, the method comprising: a preparation step of preparing the tubefitting; and a coating step of forming a resin coating layer upon acoated region provided upon the surface of the tube fitting; wherein:the coating step includes a dipping step of dipping the tube fittinginto a coating material that includes as components a polyethylene basedsubstance, a lubricant, and solid particles and having viscosity withina range of 4.24 to 5.27 mPa·s at 25° C. measured using a rotationalviscosimeter conforming to the ISO 2555:1990 standard with therotational speed of the spindle of the rotational viscosimeter set to 60rpm, and adhering the coating material to the coated region, and adrying step of, after the dipping step, drying the coating materialwhich is adhered to the coated region of the tube fitting; and in thedipping step, with mass per unit area w (g/m²) being defined as being avalue obtained by dividing the mass difference between a state with theresin coating layer and a state without the resin coating layer, themass per unit area w is controlled within the range 0.79<w<10.07 bydipping the tube fitting while regulating the temperature of the coatingmaterial within a range of 30° C. to 40° C.

According to the above aspect, by selecting a coating material whoseviscosity comes within the viscosity range described above, and byadjusting the temperature of this coating material to be within thetemperature range described above, it is possible to control the massper unit area w so that the relation 0.79<w<10.07 is satisfied.

In the above aspect, in the drying step, the mass per unit area w may becontrolled to be within the range 0.79<w<10.07 by drying the coatingmaterial while rotating the tube fitting to which the coating materialis adhered at a predetermined rotational speed. Since it is possible todry the coating material while shaking off the excess coating materialby centrifugal force, it is possible to control the mass per unit area win an appropriate manner.

An axial force measurement device according to an aspect of theinvention disclosed is an axial force measurement device which includes:a frame; a tightening actuation unit that performs tightening operationupon a screw type tube fitting installed upon a tube upon which isformed an annular portion projecting in the radially outward directionfrom the tube; and a mating member holding unit that holds a testingmember into which the tube fitting is screwed and that is coupled to thetube, with the tightening actuation unit and the mating member holdingunit being provided to the frame in line along the direction of areference axis that extends in the direction of the center line of thetube fitting and the tube axis of the tube, wherein the testing memberhas: a first part through which is pierced a screw hole in which aninternal thread that engages with the tube fitting is formed and asecond part having a bottom portion against which the annular portion ofthe tube is pressed; and the mating member holding unit holds the firstpart and the second part in a state in which the screw hole of the firstpart and the bottom portion of the second part are concentricallyabutted against one another, and wherein one of the first part and thesecond part is held by the mating member holding unit in a state inwhich it cannot shift along the direction of the reference axis, whilemovement of the other of the first part and the second part isrestricted in the direction of the reference axis in a state in which aload cell that outputs a signal corresponding to the load in thereference axis direction is interposed.

Generally, the axial force of a screw type tube fitting is calculated bydividing the tightening torque by the torque coefficient and the nominaldiameter of the screw. Since the torque coefficient is not constant andis set to an appropriate value by experience, the axial force that iscalculated is only an approximate value. Due to this, in order toacquire the axial force of the tube fitting accurately, a device thatcan measure the axial force of the tube fitting directly is required.According to this axial force measurement device, when the tube fittinginstalled upon the tube is screwed into the member for testing, the tubefitting advances while engaging with the internal thread of the firstpart, and the annular portion presses against the second part. Due tothis, a separating force acts upon the first part and the second part topull them apart in the direction of the reference axis. One of the firstpart and the second part is held so that it cannot shift in thedirection of the reference axis, while movement of the other one of thefirst part and the second part is restricted in the direction of thereference axis in a state that a load cell is interposed. Due to this,the load in the direction of the reference axis that acts upon thatother one of the first part and the second part corresponds to thereaction force of the axial force of the tube fitting. Accordingly, itis possible directly to acquire the axial force of the tube fitting as ameasurement value on the basis of the output signal of the load cell,irrespective of any calculation based upon the tightening torque.

In this aspect, the mating member holding unit may have a first jigthat, along with holding either one of the first part and the secondpart, is fixed to the frame, and a second jig of which movement, alongwith holding the other one of the first part and the second part, isrestrained in the direction of the reference axis with the load cellbeing imposed.

A measurement method of axial force according to an aspect of theinvention disclosed is a measurement method of axial force of a tubefitting, where the axial force generated upon a screw type tube fittingis measured, the tube fitting being screwed into a testing member in astate that the tube fitting is installed upon a tube upon which isformed an annular portion projecting radially outward from the tube, themeasurement method including: a first step of providing as the testingmember, a first part through which is pierced a screw hole in which aninternal thread that engages with the tube fitting is formed and asecond part having a bottom portion against which the annular portion ofthe tube is pressed in line along a direction of a reference axisextending in a direction of a tube axis of the tube, so that the screwhole and the bottom portion are in a concentric state; a second step ofholding one of the first and second parts provided in line along thedirection of the reference axis in a state in which the one of the firstand second parts cannot shift in the direction of the reference axis,and of screwing into the first part, the tube fitting installed upon thetube in a state that the other one of the first and second parts isrestricted in the direction of the reference axis; and a third step ofmeasuring as the axial force of the tube fitting, a load imposed uponthe other one of the first and second parts in the direction of thereference axis.

The invention claimed is:
 1. A tube fitting that is capable of couplinga tube made from metal and provided with an annular portion upon an endportion thereof, to a mating member by being installed upon an externalperiphery of the tube and by being fastened to the mating member in astate of contacting the annular portion, the annular portion projectingin a radially outward direction from the tube, the tube fittingcomprising: a threaded portion upon which is formed an external threadthat engages with an internal thread provided in the mating member; acontact portion that is provided at an end portion of the tube fittingin a direction of progression of the external thread when the tubefitting is being fastened, and that, when the tube fitting is fastenedto the mating member, contacts the annular portion to press the annularportion against the mating member; and a resin coating layer providedover a coated region that includes surfaces of the threaded portion andthe contact portion; and wherein: a through hole extending in adirection parallel to the direction of progression is pierced throughboth the threaded portion and the contact portion; the external threadhas an outer diameter of 9.53 to 14.0 mm, and the contact portion has aninner diameter of 4.98 to 8.44 mm; the resin coating layer includes apolyethylene based substance, a lubricant, and solid particles; athickness of the resin coating layer is not uniform throughout thecoated region; and when mass per unit area w (g/m²) is defined as beinga value obtained by dividing a mass difference between a state with theresin coating layer and a state without the resin coating layer by asurface area of the coated region, the mass per unit area w satisfies arelation 0.79<w<10.07.
 2. The tube fitting according to claim 1, whereinthe mass per unit area w satisfies a relation 1.20<w<10.07.
 3. The tubefitting according to claim 1, wherein the mass per unit area w satisfiesa relation 0.79<w<9.00.
 4. The tube fitting according to claim 1,wherein the mass per unit area w satisfies a relation 0.79<w<7.50. 5.The tube fitting according to claim 1, wherein the mass per unit area wsatisfies a relation 0.79<w<6.00.
 6. The tube fitting according to claim1, wherein, when a testing member corresponding to the mating member anda testing tube having a same outer diameter as the tube and having atesting annular portion corresponding to the annular portion areprepared, and when a fastening test is repeated n times (where 1<n<6),the fastening test including: fastening operation to fasten the tubefitting to the testing member with a predetermined tightening torque, ina state that the contact portion is contacted against the testingannular portion; and release operation to release coupling of thetesting tube from the testing member by loosening a fastening stateafter the fastening operation, if a maximum axial force generated in afirst performance of the fastening test is termed an initial axial forceF₁ (kN) and a maximum axial force generated in an n-th performance ofthe fastening test is termed an n-th axial force F_(n) (kN), and if avalue obtained by −(F_(n)−F₁)/(n−1) is defined as being an axial forcedecrease ratio α (kN/turn), then the axial force decrease ratio αsatisfies a relation 0<α<1.75.
 7. The tube fitting according to claim 6,wherein the tube is a brake tube that is employed as a brake conduit foran automobile, and, when the predetermined tightening torque is in arange of 12.0 to 22.0 Nm, the initial axial force F₁ satisfies arelation F₁<14.0.
 8. The tube fitting according to claim 1, wherein: azinc based plated layer is formed upon the surfaces of the threadedportion and the contact portion respectively, and the resin coatinglayer is provided over the zinc based plated layer; and a frictionalcoefficient of the resin coating layer is smaller than a frictionalcoefficient of the surfaces formed by the zinc based plated layer. 9.The tube fitting according to claim 8, wherein a zinc-nickel alloyplated layer is provided as the zinc based plated layer.
 10. The tubefitting according to claim 1, wherein the resin coating layer is made byadhering coating material including the polyethylene based substance,the lubricant, and the solid particles to the coated region, and aviscosity of the coating material is in a range of 4.24 to 5.27 mPa·s ata temperature of 25° C. measured using a rotational viscosimeter havinga spindle and conforming to the ISO 2555:1990 standard, a rotationalspeed of the spindle being set to 60 rpm.
 11. The tube fitting accordingto claim 1, wherein: the tube is provided with a bent portion at aposition remote from the annular portion; the tube fitting that isinstalled upon the external periphery of the tube so as to be preventedfrom coming off by the annular portion and the bent portion; and theresin coating layer includes a polyethylene based substance, alubricant, and solid particles.
 12. The tube fitting according to claim11, wherein the mass per unit area w satisfies a relation 1.20<w<10.07.13. The tube fitting according to claim 11, wherein the mass per unitarea w satisfies a relation 0.79<w<9.00.
 14. The tube fitting accordingto claim 11, wherein the mass per unit area w satisfies a relation0.79<w<7.50.
 15. The tube fitting according to claim 11, wherein themass per unit area w satisfies a relation 0.79<w<6.00.
 16. The tubefitting according to claim 11, wherein: the tube is a brake tube that isemployed as a brake conduit for an automobile; and when a testing membercorresponding to the mating member and a testing tube having a sameouter diameter as the tube and having a testing annular portioncorresponding to the annular portion are prepared, and when a fasteningtest is repeated n times (where 1<n<6), the fastening test including:tightening operation to fasten the tube fitting to the testing memberwith a tightening torque in the range of 12.0 to 22.0 Nm, in a statethat the contact portion is contacted against the testing annularportion; and release operation to release coupling of the testing tubefrom the testing member by loosening a fastening state after thetightening operation, if a maximum axial force generated in a firstperformance of the fastening test is termed an initial axial force F₁(kN) and a maximum axial force generated in an n-th performance of thefastening test is termed an n-th axial force F_(n) (kN), and if a valueobtained by −(F_(n)−F₁)/(n−1) is defined as being an axial forcedecrease ratio α (kN/turn), then the initial axial force F₁ satisfies arelation F₁<14.0 and the axial force decrease ratio α satisfies0<α<1.75.
 17. The tube fitting according to claim 12, wherein: the tubeis a brake tube that is employed as a brake conduit for an automobile;and when a testing member corresponding to the mating member and atesting tube having a same outer diameter as the tube and having atesting annular portion corresponding to the annular portion areprepared, and when a fastening test is repeated n times (where 1<n<6),the fastening test including: tightening operation to fasten the tubefitting to the testing member with a tightening torque in the range of12.0 to 22.0 Nm, in a state that the contact portion is contactedagainst the testing annular portion; and release operation to releasecoupling of the testing tube from the testing member by loosening afastening state after the tightening operation, if a maximum axial forcegenerated in a first performance of the fastening test is termed aninitial axial force F₁ (kN) and a maximum axial force generated in ann-th performance of the fastening test is termed an n-th axial forceF_(n) (kN), and if a value obtained by −(F_(n)−F₁)/(n−1) is defined asbeing an axial force decrease ratio α (kN/turn), then the initial axialforce F₁ satisfies a relation F₁<14.0 and the axial force decrease ratioα satisfies 0<α<1.75.
 18. The tube equipped with a tube fittingaccording to claim 16, wherein: a zinc based plated layer is formed uponthe surfaces of the threaded portion and the contact portionrespectively; the resin coating layer is provided over the zinc basedplated layer; a zinc-nickel alloy plated layer is provided as the zincbased plated layer; and a frictional coefficient of the resin coatinglayer is smaller than a frictional coefficient of the surfaces formed bythe zinc based plated layer.
 19. The tube fitting according to claim 1,wherein: the tube is a brake tube that is employed as a brake conduitfor an automobile; when a testing member corresponding to the matingmember and a testing tube having a same outer diameter as the tube andhaving a testing annular portion corresponding to the annular portionare prepared, and a fastening test is repeated n times (where 1<n<6),the fastening test including: fastening operation to fasten the tubefitting to the testing member with a tightening torque in a range of12.0 to 22.0 Nm, in a state that the contact portion is contactedagainst the testing annular portion; and release operation to releasecoupling of the testing tube from the testing member by loosening afastening state after the fastening operation, if a maximum axial forcegenerated in a first performance of the fastening test is termed aninitial axial force F₁ (kN) and a maximum axial force generated in ann-th performance of the fastening test is termed an n-th axial forceF_(n) (kN), and if a value obtained by −(F_(n)−F₁)/(n−1) is defined asbeing an axial force decrease ratio α (kN/turn), a range of the mass perunit area w is set so as to satisfy relations: F₁<14.0 and 0<α<1.75. 20.The tube fitting according to claim 19, wherein a range of the mass perunit area w is 1.20<w<10.07.